1. Field of the Invention
This invention relates generally to an apparatus and a method for tuning a radio receiver to the strongest signals of a broadcast radio spectrum, and more particularly to automatic tuning circuits in electronically tuned AM radios for selecting and automatically locking onto the strongest portion of a received AM radio signal having a signal strength above a preselected threshold level.
2. Description of the Prior Art
In conventional radio signal receivers, the output signal of the receiver radio is generally of better quality when the receiver is tuned as close as possible to the carrier or center frequency of the transmitted frequency signal, which the operator or listener desires to receive. It is well known that when an AM or FM receiver, for example, is tuned to a frequency somewhat removed from the broadcast frequency of the selected station or channel, a significant amount of undesired noise and distortion will be present in the audio output of the receiver. As the radio's receiver is tuned to frequencies closer to the broadcast frequency, the audio output becomes less noisy (i.e., the signal-to-noise ratio increases). Distortion decreases until the optimum is attained at or very near the broadcast frequency.
In manually tuned radio frequency signal receivers, the tuning mechanism may be adjusted until an audio signal is produced which is acceptable to the listener. A device such as a signal strength meter may be provided to visually aid in the manual tuning thereof. Another conventional tuning aid is a squelch muting circuit which, as its name implies, prevents the receiver from producing an audio output signal unless the strength of a received signal exceeds a predetermined level.
In the type of radio generally referred to as having a signal seeking or scanning receiver, a scanning circuit is typically provided which varies the frequency of the local oscillator either continuously or in discrete increments. A detector circuit is often used to stop the scanning operation when a signal of sufficient strength is received. Conventional detector circuits of this type are manually adjustable in order to compensate for the differences in the strength of the various received signals. Such differences in signal strength can be attributed to a number of factors, including differences in distances and orientation between the transmitting antenna and receiving antenna, as well as differences in the transmission power of different radio broadcasting stations. It is difficult to preset such a detector circuit so that it will provide optimum tuning for all signals of interest.
In prior art radios, a compromise was often made by broadening the range (e.g., lowering the threshold signal level which the radio will lock to) of the detector or squelch circuit. However, such a detector circuit permitted the scanning receiver to stop at a frequency other than the broadcast frequency of a radio signal. If, for example, an electronically tuned radio that scanned in increments of 5 KHz encountered a sufficiently strong radio signal at a receiver frequency of 605 KHz, its detector circuit would stop the receiver at 605 KHz. However, the signal at 605 KHz may have resulted from a strong radio signal broadcast at 610 KHz. In this case, the receiver would have stopped at 605 KHz before it reached the center tuned signal at 610 KHz. In other words, as the scanning receiver was tuned near a strong signal, the threshold signal level for stopping the scanning action could have been reached before the scanner was tuned to the broadcast frequency of the signal. Moreover, if the scanning receiver displayed the tuned frequency, the frequency displayed was not the broadcast frequency. Thus, it is desirable to stop at only center tuned signals, and not to stop at signals that are not center tuned.
Additional problems existed with prior art radios. First, none of the circuits provided for a tunable resonator for tuning to the broadcast frequency of the radio signal. Second, existing technology required many individual discrete electrical components to be properly selected and connected together to obtain a received signal of the quality desired. Third, signal seeking or scanning receivers have been successfully developed for FM radio broadcast signals, but AM signal seeking or scanning receivers have been or marginal quality. Fourth, prior art radio technology has failed to solve problems associated with AM radio systems, such as (1) removal of the audio component from the modulated component of the IF signal, and (2) elimination of the tweet interference. Fifth, AM radio systems have not advantageously used the amplitude characteristic of a tuned element, as FM radio systems have used the phase characteristic of a tuned element.